Post-transit photocurrent spectroscopy studies on the gap state distribution in hydrogenated amorphous silicon: Trap occupation problems in leaky junctions and high-defect-density samples

Abstract

Post-transit photocurrent spectroscopy (PTPS) has been employed as an effective method to obtain the energy distribution of gap states in undoped hydrogenated amorphous silicon. In this article, we demonstrate that the simplifying assumptions in conventional PTPS related to the trap occupation are not always valid and special care should be taken when we extend PTPS to samples with larger leakage currents and/or high-defect densities. We show test diode structures and experimental conditions to exactly determine the energy distribution of both electron and hole trap states in these cases. The attempt-to-escape frequency, ν, both for electrons and holes has been evaluated from the temperature dependence of the characteristic points in PTPS signal. It has been found that the density of electron and hole trap states in the deeper energy range increases with long exposure to visible light while the density in the shallower energy range decreases, which suggests the occurrence of defect conversion.